The sponsor's administrative charge, independent legal advice on both the immigration case and the investment, source-of-funds preparation, government amounts from the current USCIS fee schedule, translations, and the later petition to remove conditions.
Pay for advice that is not selling you the investment
Hypothetical example: the couple rely on the sponsor's recommended lawyer for everything and never obtain an independent view of the offering. That saves a fee and removes the only check on the largest financial decision of their lives. Budget for two separate advisers: immigration counsel for the case, and counsel or an experienced adviser to review the investment documents on their merits.
Add the sponsor's administrative charge, asking what portion is refundable and when; government amounts per the current G-1055 schedule for each filing; accountant time to assemble the source-of-funds record; certified translations; and the later removal-of-conditions filing with its own legal cost. Verify all current official amounts on the day of the decision rather than relying on figures from an earlier summary.